Low rank carbonaceous materials, such as brown coal, peat and lignite, are materials having water locked into a microporous carbonaceous structure. The water content is typically high—for example 60% or higher. This means that such materials have a low calorific value. Moreover, these materials have the undesirable mechanical properties of being soft, friable and of low density, meaning that they are difficult, messy and inconvenient to handle.
Prior process for upgrading low rank carbonaceous materials (which for ease of discussion will be hereinafter collectively referred to as “brown coal”) have included “briquetting” and solar drying.
Briquetting typically involves heating the raw brown coal to remove excess water, then pressing the cooled brown coal into briquettes using an extrusion press or roll briquetting machine. However, briquetting is an expensive process due to the requirement for thermal energy and the mechanical wear on the extrusion press or roll briquetting machine.
The solar drying process involves milling of the brown coal with addition of water for long periods (e.g. up to 16 hours), then solar drying of the milled slurry in shallow ponds. This process is lengthy—particularly the solar drying step which may take up to several months—and energy intensive.
Another proposal mechanically releases water from brown coal by physically breaking up the coal. However, this process is inconvenient and time consuming and still requires lengthy air drying of the final product.
Another prior art process is the subject of WO 01/54819 in the name of applicant, the entire disclosure of which is incorporated herein by reference. In that process, brown coal is subjected to shearing-attritioning to form a plastic mass which is then extruded into pellets and air dried.
While a number of upgrading processes have been developed to upgrade brown coal, it has been to date difficult to accommodate the natural variations in the characteristics of brown coal deposits in those processes. Brown coal occurs in a number of deposits around the world, including Australia, USA, Germany, Poland, Indonesia and India. These deposits vary considerably in composition such as in water content and ash content. Even within a specific deposit there are compositional variations. This is particularly relevant for water content which can vary significantly depending on the location within the deposit and even when a particular sample of brown coal is taken (eg, water content of brown coal typically is higher after rain). This variation in composition has implications for the upgrading process conditions selected for the brown coal mined from the deposit. This means that it can be difficult to standardise the process steps used in a particular upgrading process.
It is accordingly an object of the present invention to provide a process for upgrading brown coal which overcomes, or at least alleviates, one or more disadvantages of the prior art.